THE RELATION OF CLIMATE TO POMOLOGY 247 



through its natural phenomena, such as blossoming, putting 

 out leaves, and the like. The formulation of such laws is 

 not surprising, and they were at first widely accepted ; how- 

 ever, there are several fundamental objections to them. 



Hoffmann suggested that the time of bloom, as well as the 

 other natural phenomena in the seasonal development of 

 the tree, is dependent on the sum total of heat available to 

 the plant up to that event. His zero point was arbitrarily 

 fixed at the first of January after which he took the sum of 

 the daily maximum positive temperatures (above 32° F.) 

 of a thermometer exposed to the sun up to the beginning 

 of the event. In this way, he accounted to a large extent for 

 the difference in time of blooming of different seasons. 



De CandoUe and others have fixed the temperature at 

 which the plant becomes active at 43° F. (6° C.) instead of 

 32° F., but even this ignores the well-established fact that 

 plants have different optimum temperatures. It also may 

 be assumed that the separate phases of the plant activities 

 may have different optima. 



Furthermore, it has been pointed out that the buds of the 

 tree may be much further advanced when they enter the 

 winter condition some seasons than others, due to the sea- 

 sonal variations. This would probably modify the total 

 amount of heat necessaiy for blossoming the ensuing sea- 

 son. Hence, it seems impractical to lay down any physio- 

 logical constant for plants. 



215. The blooming season. — As indicated above, fruit- 

 trees of any given variety do not bloom at exactly the same 

 time year after year, but may be a few days earlier or later 

 than the average, depending largely on the character of the 

 season. In an "early" season, they will bloom earlier and 

 in a later season the blooming will be retarded. In addition 

 to the total temperature available, other factors must also 

 play a part in causing the difference, such as the intensity of 



